For the purpose of improving the fatigue strength and/or wear resistance of a machine component, a nitrogen-enriched layer having a high nitrogen concentration as compared to the interior may be formed at the surface layer of the machine component by means of carbonitriding or the like. In a general carbonitriding process, atmosphere gas is often used, which is based on producing carrier gas by mixing propane, butane or city gas with air at a high temperature greater than or equal to 1000° C. (endothermic converted gas: hereinafter, referred to RX gas) (RX is a trademark of Surface Combustion, Inc.), having a small amount of propane, butane, and ammonia added. By heating a workpiece in this atmosphere gas, a nitrogen-enriched layer is formed at the surface layer of the workpiece. During a carbonitriding process using RX gas as the carrier gas, a nitriding response occurs by undecomposed ammonia (for example, refer to “Improvement of Wear Resistance by Heat Treatment for Carburizing Steel” by Nobuyuki Mouri et al., NTN TECHNICAL REVIEW, 2008, No. 76, pp 17-22 (NONPATENT DOCUMENT 1)).